FoldingSet is implemented as a chained hash table. When there is a hash
collision during insertion, which is common as we fill the table until a
load factor of 2.0 is hit, we walk the chained elements, comparing every
operand with the new element's operands. This can be very expensive if the
MDNode has many operands.
We sacrifice a word of space in MDNode to cache the full hash value, reducing
compares on collision to a minimum. MDNode grows from 28 to 32 bytes + operands
on x86. On x86_64 the new bits fit nicely into existing padding, not growing
the struct at all.
The actual speedup depends a lot on the test case and is typically between
1% and 2% for C++ code with clang -c -O0 -g.
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StringMap. This was redundant and unnecessarily bloated the MDString class.
Because the MDString class is a "Value" and will never have a "name", and
because the Name field in the Value class is a pointer to a StringMap entry, we
repurpose the Name field for an MDString. It stores the StringMap entry in the
Name field, and uses the normal methods to get the string (name) back.
PR12474
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not part of the IR, are not uniqued, and may be safely RAUW'd.
This replaces a variety of alternate mechanisms for achieving
the same effect.
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Make MDNode::destroy private.
Fix the one thing that used MDNode::destroy, outside of MDNode itself.
One should never delete or destroy an MDNode explicitly. MDNodes
implicitly go away when there are no references to them (implementation
details aside).
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Tested: clang debug bootstrap, llvm-gcc bootstrap, `make check-lit`
after configuring with --with-llvmgccdir (and this did run the
FrontendC* tests this time)
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after the MDNode in memory. This eliminates the operands
pointer and saves a new[] per node.
Note that the code in DIDerivedType::replaceAllUsesWith is wrong
and quite scary. A MDNode should not be RAUW'd with something
else: this changes all uses of the mdnode, which may not be debug
info related! Debug info should use something non-mdnode for
declarations.
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getMDKindID/getMDKindNames methods to LLVMContext (and add
convenience methods to Module), eliminating MetadataContext.
Move the state that it maintains out to LLVMContext.
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why one was replaced with the other. Even in the specific case of
debug information, it doesn't make sense to transfer the location over,
this will just result in jumbled loc info.
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a convention (shadowing the setter with private forwarding function) to
prevent subclasses from accidentally using it.
This exposed some bogosity in ConstantExprs, which was propaging the
opcode of the constant expr into the NUW/NSW/Exact field in the
getWithOperands/getWithOperandReplaced methods.
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I asked Devang to do back on Sep 27. Instead of going through the
MetadataContext class with methods like getMD() and getMDs(), just
ask the instruction directly for its metadata with getMetadata()
and getAllMetadata().
This includes a variety of other fixes and improvements: previously
all Value*'s were bloated because the HasMetadata bit was thrown into
value, adding a 9th bit to a byte. Now this is properly sunk down to
the Instruction class (the only place where it makes sense) and it
will be folded away somewhere soon.
This also fixes some confusion in getMDs and its clients about
whether the returned list is indexed by the MDID or densely packed.
This is now returned sorted and densely packed and the comments make
this clear.
This introduces a number of fixme's which I'll follow up on.
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